Bacterial Blight in Carrot Seed Crops in the Pacific Northwest

Carrot (Daucus carota subsp. sativus) seed crops in Oregon and Washington were surveyed in 2001-02 and 2002-03 for development of Xanthomonas campestris pv. carotae, causal agent of bacterial blight. For each state and season, 20 plants were sampled from each of 7 to 12 direct-seeded crops twice in the fall or winter and three times from spring to summer; and from each of 2 to 4 steckling (root-to-seed) crops three times from spring to summer. X. campestris pv. carotae was detected in 1 of 15 and 6 of 32 stock seed lots planted in the fall in Oregon and Washington, respectively, and in 2 of 6 steckling shipments planted in each state in the spring. The pathogen was detected at 10 2 o 10 8 CFU/g foliage in 1 of 22 and 10 of 14 direct-seeded crops within 2 months of planting in 2001 and 2002, respectively. The prevalence of X. campestris pv. carotae then increased through the season in most seed crops, although bacterial blight symptoms were not observed until April in Oregon and July in Washington in both seasons. In August 2002 and 2003, X. campestris pv. carotae was detected in all 10 and 13 crops surveyed in Oregon, respectively; and in 11 of 12 and 7 of 10 crops in Washington, respectively. The pathogen was typically less prevalent in steckling versus direct-seeded crops. X. campestris pv. carotae was detected in 20 of 22 and 19 of 23 harvested seed lots in Oregon and Washington, respectively, at populations ranging from 1.3 × 10 1 o 1.4 × 10 8 CFU/g seed. Airborne X. campestris pv. carotae, detected ≤1,600 m downwind of crops being threshed in Oregon in September of 2003 and 2004, may provide a source of inoculum for newly planted seed crops between overlapping biennial seasons for carrot seed production. Despite the prevalence of this pathogen in the Pacific Northwest, carrot seed lots free of X. campestris pv. carotae were detected, demonstrating the ability to produce clean seed in this region by adhering to recommended practices for management of bacterial blight.